1. Technical Field
The invention concerns a method for manufacturing an optical cable comprising a metal tube in which at least one optical fiber waveguide is located, wherein a metal band drawn from a supply reel is gradually shaped into a slotted tube, the optical fibers drawn from another supply spool are introduced into the still open slotted tube, the lengthwise slot of the slotted tube is welded, the outside diameter of the welded tube is reduced and the metal band, the optical fibers and the welded tube are transported by a storage reel.
2. Description of the Prior Art
A method for manufacturing such cables is known from EP 0 229 123 B1.
The manufacture of such optical cables involves the significant problem of economically producing an overlength of the optical fibers inside the cable. This overlength is necessary because of the very different thermal expansion coefficients of the metal and glass materials. To prevent mechanical stress of the optical fibers in the event the metal tube exposed to the elevated temperatures, the overlength should not be too small. On the other hand, the overlength should not be too large, since an unacceptable compression of the optical fibers could result when the metal tube is exposed to low temperatures, which leads to an increase in attenuation. The optical fibers must therefore contain an overlength which takes both aspects into consideration, so that unallowable stress conditions cannot occur under any circumstances.
With the method according to EP 0 229 123 B1, the overlength is obtained by introducing the optical fibers with a pushing device into the still open slotted tube at a higher speed than the slotted tube. According to an alternative, the optical fibers are guided by a filler tube to the storage reel area, where they are blown by flowing gas onto the outside surface of the metal tube.
With a method described by EP 0 456 836 A1, the drawn metal band contains a prestress and is thereby elastically strained.
In both methods, an accurate overlength of the optical fibers inside the metal tube is difficult to obtain.